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The Atom Objectives: 1) Describe the characteristics of protons, neutrons and electrons. 2) Describe and draw the patterns of electrons for the first 20 elements in the periodic table using BohrRutherford diagrams. • All matter on Earth is composed of one or more of the 118 elements on the periodic table • Elements are made up of atoms RECALL Rutherford proposes that the atom contained a central nucleus with a small, dense, positive charge in the center. James Chadwick proposed that the nucleus of the atom contains protons and neutral particles called neutrons. Mass of a neutron is about the same as the mass of a proton and a neutral atom has the same number of protons and electrons. Niels Bohr 1913 Studied under Rutherford. Electrons are in orbits. Rather like planets orbiting the sun. With each orbit only able to contain a set number of electrons. Bohr’s Atom electrons in orbits nucleus We Now Know that …. • An atom is the smallest particle of an element that still has the same properties of that element • Atoms are incredibly small – about a million times smaller than the thickness of a human hair • They are made up of even smaller subatomic particles: protons, neutrons, and electrons HELIUM ATOM Shell proton + - N N + electron What do these particles consist of? - neutron Atomic Structure Particle Charge Name Location Mass Symbol proton + nucleus Heavy p+ neutron neutral nucleus Heavy n0 electron - energy levels outside the nucleus Light e- In Summary •Protons have a positive (+) electrical charge, and are found in the nucleus (centre) of an atom •Neutrons are electrically neutral and are also found in the nucleus of an atom •Electrons have a negative (-) electrical charge and found in energy levels around the nucleus What makes atoms different from each other? • Each element or atom has its very own number of protons • For example, calcium has 20 protons, while magnesium has 12 • The number of protons in an atom is called the atomic number • The periodic table is arranged based on the atomic number of each element • In a neutral atom, the number of protons equals the number of electrons • # protons = # electrons • Therefore calcium has 20 electrons, while magnesium has 12 How we determine the number of neutrons? • The mass number is the total number of protons and neutrons in an atom • mass number = # protons (or atomic number) + # neutrons • Since we always know the number of protons (atomic number on the periodic table), we can subtract that value from the mass number to get the number of neutrons • # neutrons = mass number – atomic number (or # protons) Example of ATOMIC STRUCTURE 2 He 4 Atomic number the number of protons in an atom Atomic mass the number of protons and neutrons in an atom number of electrons = number of protons Practice Element Symbol Helium He Atomic Number 2 magnesium 4 Protons Electrons Neutrons 2 2 2 24 26 zinc Mass Number 56 35 Bohr-Rutherford Diagrams of the Atom p+ no Bohr-Rutherford Diagrams of the Atom • A Bohr-Rutherford Diagram is a drawing that shows the number of protons, neutrons and electrons for the first 20 elements. • In Bohr-Rutherford diagrams, each energy level is shown as a ring around the nucleus • Evidence indicates that only a limited number of electrons can exist in each energy level ATOMIC STRUCTURE Electrons are arranged in Energy Levels or Shells around the nucleus of an atom. • first shell a maximum of 2 electrons • second shell a maximum of 8 electrons • third shell a maximum of 8 electrons • fourth shell a maximum of 2 electrons How to draw a Bohr-Rutherford diagram of an atom: • Determine the number of protons, electrons, and neutrons. • Draw the nucleus and indicate the number of protons and neutrons. • Add the electrons one at a time, starting with the first energy level (the ring closest to the nucleus) and working your way out. • • • • Remember! # protons = the atomic number # electrons = # protons # neutrons = mass number (round off the atomic mass) - # protons Example #1 Hydrogen Example #2 Sodium SUMMARY 1. The Atomic Number of an atom = number of protons in the nucleus. 2. The Atomic Mass of an atom = number of Protons + Neutrons in the nucleus. 3. The number of Protons = Number of Electrons. 4. Electrons orbit the nucleus in shells. 5. Each shell can only carry a set number of electrons.